Toxigenic lipids are produced when rat liver microsomes undergo peroxidative decomposition in vitro. These lipids are stable in organic solvents. They can be partially purified by adsorption onto silicic acid followed by elution with methanol. Biological activity, detected in a red cell hemolysis test, elutes with lipid phosphate and peroxy function. The active material appears to be a mixture of peroxidized phospholipids. Biologically active mixtures are being further purified by high pressure liquid chromatography. Fatty acids, cleaved from the peroxidized phospholipids by snake venom phospholipase A2 are remarkably potent in increasing tension developed in isometric contraction of strips of smooth muscle from the fundus of the stomach of hamster or rat.